1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method, a heat treatment apparatus, and a heat treatment method using the apparatus. Specifically, the invention relates to a heat treatment method and a heat treatment apparatus for crystallizing an amorphous semiconductor film, for recrystallizing and activating a semiconductor film that has been brought back to an amorphous state by ion implantation or ion doping, and for gettering of a metal element remaining in the semiconductor film. The invention also relates to a semiconductor device manufacturing method using this heat treatment method.
2. Description of the Related Art
Having silicon as its main ingredient, a crystalline semiconductor film formed at a process temperature of 600xc2x0 C. or lower is often called low temperature polysilicon. The primal use of this crystalline semiconductor film is of as an active layer for forming a channel formation region, a source or drain region and the like in a thin film transistor (hereinafter referred to as TFT). The TFT is formed on a glass substrate to manufacture a liquid crystal display device, and technologies related thereto are receiving attention most.
Technologies for manufacturing a TFT using the above crystalline semiconductor film characteristically employ laser annealing and ion doping, and these techniques make it possible to form an n-channel TFT and a p-channel TFT on a large area glass substrate to build an integrated circuit having the CMOS structure.
A TFT needs, in addition to an impurity region of n type or p type conductivity to serve as a source and drain region, a low concentration impurity region that serves as LDD (lightly doped drain) useful in reducing leak current and in stabling TFT characteristics. The TFT also needs to be doped with an impurity element of one conductivity type in order to control the threshold voltage. Formation of the low concentration impurity region and the doping with an impurity element of one conductivity type are conducted through ion doping and subsequent activation treatment. In this ion doping, all of ion species generated are implanted by acceleration implantation without being subjected to mass separation.
The impurity is activated after doping by furnace annealing, laser annealing, or RTA (rapid thermal annealing). The source and drain region, which is relatively heavily doped (concentration: 1015 atoms/cm2), is said to need a rather high temperature and take a longer time to activate the impurity element used in doping for enhancement of conductance. Laser annealing melts a semiconductor film and is unsatisfactory in controllability and reproducibility, which makes it difficult to employ laser annealing in a mass production process. On the other hand, furnace annealing is suitable for a mass production process since it treats in batches but its activation efficiency decreases as the process temperature becomes low to prolong the treatment time, which is a problem.
Laser annealing as a crystallization technique is capable of forming a crystalline semiconductor film on a glass substrate. However, the reaction is of nonequilibrium one and the crystals formed therefore have small grain size and many defects. Laser annealing crystallization has little direct control factors other than the energy density and irradiation number of laser light and the substrate heating temperature. Furthermore, the direct control factors are effective only for limited ranges. For example, control by energy density is effective when it is 250 to 400 mJ/cm2 and it only gives an amorphous structure outside the range.
A crystallization technique involving adding with a metal element is disclosed in Japanese Patent Application Laid-open No. Hei 7-183540 as a method that can provide better crystals than those obtained by laser annealing. The metal element used is nickel, palladium, lead, or the like. Various methods can be employed in the doping, such as plasma treatment, evaporation, ion implantation, solution application, and sputtering. Crystallization is made by heat treatment at 500 to 600xc2x0 C., preferably 550xc2x0 C. for four hours. However, this method leaves the metal element in the semiconductor film crystallized and often needs gettering. Most of the remaining metal element forms a deep level in the forbidden band of the semiconductor film to act as a lifetime killer and to cause an increase of leak current in the junction.
Gettering using phosphorus can make the metal element segregate in a region doped with phosphorus. This gettering uses an annealing furnace and requires heat treatment typically at 450 to 600xc2x0 C. for twelve hours. The metal element thus segregates in the phosphorus-doped region.
One of the most promising application fields for the thus manufactured TFT is liquid crystal displays and other flat panel displays. In the flat panel display field, increasing the substrate size is demanded for improvement of productivity. These larger substrates come in various sizes and 960xc3x971100 mm2 is among them. A substrate measuring 1000 mm or more in one side is more often considered than others. Liquid crystal display devices are not the only ones who face this demand but it is a common object to all large-area integrated circuits constructed from TFTs formed on glass substrates.
In order to improve the productivity, it is necessary to reduce the number of steps in a TFT manufacturing process and shorten the treatment time. Then, a furnace annealing apparatus, which treats in batches, would not be helpful in improving the production efficiency. If the furnace annealing apparatus is enlarged, current consumption is increased for heating the large capacity furnace, not to mention it needs larger installment area.
RTA is suitable in terms of productivity. RTA is capable of heating and raising the temperature high in a short period of time and latently has higher processing ability than furnace annealing also when the single wafer method is employed. However, short heating time means high heating temperature while a temperature exceeding the distortion point of glass, or even its softening point, is required in order to obtain desired effects in activation and gettering. For instance, a glass substrate is bent and deformed by its own weight by merely sixty seconds of heat treatment at 800xc2x0 C. for the purpose of gettering.
The present invention has been made to solve the problems above, and an object of the present invention is therefore to provide, for manufacture of a semiconductor device using a low heat resistant substrate such as a glass substrate, a method of heat treatment for activating an impurity element that is used to dope a semiconductor film and for performing gettering on the semiconductor film in a short period of time without deforming the substrate. Another object of the present invention is to provide a heat treatment apparatus for carrying out the above heat treatment, and a method of manufacturing a semiconductor device using the heat treatment apparatus.
In order to attain the above objects, the present invention provides a method of heat treatment for performing gettering and activation on a semiconductor film formed on a low heat resistant substrate such as a glass substrate in a short period of time without causing deformation or other heat-induced damage to the substrate, as well as a heat treatment apparatus for carrying out the method.
A semiconductor film is formed on a substrate and then doped with phosphorus by ion doping using phosphine without mass separation. Phosphorus getters a metal element in the semiconductor film through a mechanism inferred as follows. When the semiconductor film is selectively doped with phosphorus, a region of the semiconductor film that is doped with phosphorus (gettering region) becomes amorphous. The semiconductor film is then heated so that the amorphous gettering region is crystallized. At this point, phosphorus in the gettering region is moved into a lattice cell of the semiconductor film. The heat treatment cuts the bond of a compound formed from the metal element (referred to as metal compound) in a region that is not doped with phosphorus (to-be-gettered region) (severing of the bond will be called release). Then, the metal element moves (diffusion) to couple with phosphorus (trapping). The metal element is removed from or reduced in the to-be-gettered region supposedly in this way.
Gettering is done through three stages: one, release of the metal element from the metal compound in the to-be-gettered region, two, diffusion of the metal element, and three, trapping of the metal element by phosphorus in the gettering region. The energy required for release of the metal element is estimated as several hundreds degree (xc2x0 C.) and it is known that the metal element is readily released through heat treatment around 500xc2x0 C. When the heat treatment is conducted at higher temperature, the rate the metal element diffuses is raised but effective gettering of the metal element cannot be expected. This is probably because phosphorus is stuck in a lattice cell and prevented from coupling with the metal element when the temperature is high.
Accordingly, for effective gettering, the heat treatment has to be made at low temperature while accelerating diffusion of the metal element. The invention achieves this by heating the to-be-gettered region at a temperature higher than the temperature at which the gettering region is heated through pulsative radiation from a lamp light source. Structures of the gettering region and the to-be-gettered region are accordingly modified and a light absorbing film is formed on the gettering region. A gate electrode may serve as the light absorbing film. In this case, a part of the gate electrode is formed from, for example, a tantalum nitride film so that the tantalum nitride film is heated through the radiation from the lamp light source.
The metal element is readily released from the metal compound and diffuses into the gettering region by heating the to-be-gettered region at relatively high temperature. Then, the metal element reaches the gettering region doped with phosphorus and segregates there. High gettering effect is obtained if the heating is stopped before phosphorus is taken to a lattice cell of the silicon network and forms tetradentate bond, in other words, before activation proceeds too far.
The treatment object, namely, the semiconductor film, is heated by irradiating it several times using pulsative radiation from the lamp light source. This makes it possible to rapidly heat and rapidly cool the to-be-gettered region before heat is transmitted to the glass substrate and the gettering region. The light source could be a laser, of course, but a halogen lamp or the like is preferable considering a suitable irradiation time for activation and gettering since irradiating a large area is easy with a lamp light source. The present invention is characterized by conducting gettering and activation in this way.
As described above, the heat treatment method of the present invention involves heating a treatment object by irradiating it several times through pulsative radiation from a lamp light source, and is characterized in that radiation from the lamp light source lasts 0.1 to 20 seconds at a time and that radiation from the lamp light source is repeated several times. The method is also characterized in that the treatment object is subjected to pulsative radiation from the lamp light source such that the treatment object holds the temperature to its highest for 0.5 to 5 seconds. The method is also characterized in that the amount of coolant to be supplied is increased or reduced in accordance with blinking of the lamp light source to enhance the effect of the heat treatment on a semiconductor film that is the treatment object and to prevent a heat-induced damage to a substrate.
A heat treatment apparatus of the present invention is for carrying out the above heat treatment method and is characterized by comprising: a lamp light source; a power source for making the lamp light source blink and pulsate; a stage on which a substrate is placed; a processing chamber in which a treatment object is subjected to radiation from the lamp light source; and means for supplying a coolant to the processing chamber and increasing and reducing the amount of supply.
The lamp light source may be a halogen lamp, a metal halide lamp, a xenon lamp, a high pressure mercury lamp, a high pressure sodium lamp, or an excimer lamp.
The heat treatment apparatus of the present invention may take another structure, which is characterized by comprising: a lamp light source; a power source for making the lamp light source blink and pulsate; a stage on which a substrate is placed; a processing chamber in which a treatment object is subjected to radiation from the lamp light source; transferring means for moving the stage in one direction in the processing chamber; and means for supplying a coolant to the processing chamber and increasing and reducing the amount of supply in accordance with blinking of the lamp light source.
A semiconductor device manufacturing method of the present invention employs the above heat treatment method, and is characterized by comprising the steps of: forming a semiconductor film on a light transmissive substrate; forming an insulating film on the semiconductor film; forming a light absorptive first conductive film on the insulating film; forming a light reflective second conductive film on the first conductive film; doping the semiconductor film with an impurity of one conductivity type to form a semiconductor region of one conductivity type; and activating the semiconductor region of one conductivity type by irradiating the region several times from the light transmissive substrate side through pulsative radiation from a lamp light source.
The semiconductor device manufacturing method of the present invention may take another structure, which is characterized by comprising: a first step of forming an amorphous semiconductor film on one major surface of a light transmissive substrate; a second step of doping the amorphous semiconductor film with a metal element and then crystallizing the amorphous semiconductor film to form a crystalline semiconductor film; a third step of forming, above the crystalline semiconductor film, a conductive film so as to partially overlap the crystalline semiconductor film; a fourth step of forming a semiconductor region doped with phosphorus in the crystalline semiconductor film; and a fifth step of performing intermittent radiation from a lamp light source several times from a surface opposite to the one major surface of the light transmissive substrate.
The treatment object is kept in the coolant and is irradiated several times by radiation from the lamp light source such that the treatment object holds the temperature to its highest, 600 to 800xc2x0 C., for 30 to 600 seconds. In this way., the treatment object can be heated efficiently, completing the heat treatment. The wavelength of the electromagnetic wave radiated from the lamp light source is matched to the absorption band of the treatment object for selective heating so that the treatment object alone is heated. Specifically, a semiconductor film formed on a glass substrate having a distortion point of 700xc2x0 C. or lower is heated.